1. Field of the Invention
This invention relates generally to a canned motor pump with a high inertia flywheel and, more particularly, to a radial bearing assembly for a rotatable motor shaft on which the flywheel is mounted.
2. Background of Information
Centrifugal pumps having flywheels are well-known. The flywheel is incorporated to mechanically store kinetic energy during operation of the pump, which energy may be utilized to maintain rotation of the pump in the event of loss of motive power, such as loss of electric power. In nuclear reactors, this technology becomes very important in order to help maintain coolant circulation through the reactor core after coolant pump trip, since the nuclear fuel continues to give off substantial amounts of heat within the first several minutes after a reactor trip, and cooling is improved with forced flow. The flywheel is generally a metal disk having relatively high mass and being precisely attached to or mounted on the motor shaft for rotation therewith, the inertia of which keeps the shaft rotating after de-energization of the motor.
Pressurized water reactor (PWR) coolant pumps generally include a pump and a motor separated by a complicated shaft seal system, the seals being used as a part of the reactor coolant system pressure boundary. The seals are generally subject to about a 2500 psi pressure differential between the reactor coolant system and the containment atmosphere. These seals are susceptible to failure, and may cause a non-isolatable leak of primary coolant ranging in size from very small to fairly large. As such, seal failure may result in a challenge to the redundant safety systems provided in nuclear power plants to prevent and mitigate damage to the reactor core.
Canned pumps have been used in nuclear reactor plants for some time, and avoid the problem of the shaft seal arrangement since the entire pump, including bearings and rotor, are submerged in the pumped fluid. Therefore, the use of the pump expressly reduces the potential for a small loss of coolant accident (LOCA). Exemplary canned motor pumps are described in U.S. Pat. Nos. 3,450,056 and 3,475,631. In boiling water reactors, continued rotation of these pumps upon loss of electric power is provided by electro-mechanical means, generally in the form of a motor-generator set and typically located outside of the reactor containment for accessibility purposes, the electricity being transmitted from the generator to the pump motor through containment wall penetrations. In the event of a loss of electric power to the motor-generator set, the flywheel maintains rotation of the generator for some period of time, which continues to provide power to the pump motor. However, due to the lack of mechanical inertia in the pump itself, any localized failures of the pump or its controls may prevent the pump from extended coast-down. In addition, due to the necessity for extra equipment, this option becomes fairly expensive, both in capital cost and in operation and maintenance cost.
A flywheel within a canned or wet winding pump has been utilized. However, the losses resulting from spinning a large, high mass flywheel through the fluid contained in the pump casing are substantial. The outer surfaces of the flywheel attempt to frictionally pump the surrounding fluid, while the casing surrounding the flywheel inhibits fluid flow. Therefore, turbulent vortices form causing highly distorted fluid velocities which yields substantial drag on the flywheel. This drag is a function of the speed and area of the surface of the flywheel, which both increase with the radius of the flywheel, such drag being commonly understood to increase with about the fifth power of the diameter and about the cube of the angular velocity.
One arrangement to overcome this power loss is disclosed in U.S. Pat. No. 4,084,924 to Ivanoff et al. This patent describes a wet winding pump having a flywheel and a free-wheeling shroud rotatable relative to the shaft and the flywheel. The shroud encompasses the flywheel but is spaced apart therefrom and includes passages for ingress and egress of liquid into and out of the space between the flywheel and the shroud. This system envisions that the shroud will rotate at some angular velocity which would be approximately one-half the velocity of the flywheel, thereby creating two pumped fluid layers, one (between the flywheel and the shroud) being pumped by the flywheel, and the other (the layer outside the shroud) being pumped by the shroud. The lower relative angular velocity between the rotating surfaces therefore results in lower total drag.
A further high inertia flywheel for a canned or wet winding pump that purportedly prevents vibration of the pump, and simultaneously minimizes the losses associated with the flywheel is disclosed in U.S. Pat. No. 4,886,430 to Veronesi et al. on Dec. 12, 1989, assigned to the Westinghouse Electric Corporation. U.S. Pat. No. 4,886,430 describes a radial bearing located on the outer circumferential surface of the flywheel. The small gaps between the flywheel surface facings and the radial and thrust bearing surfaces were theorized as reducing the friction loss of the flywheel. However, testing of the flywheel and bearing arrangement described in this U.S. Pat. No. 4,886,430 showed that the expected drag reduction did not occur. Subsequent analysis revealed that close clearances, such as those in the journal bearings, increase rather than reduce drag. The analysis was proven by testing that showed a 30% drag reduction when the close clearance radial bearing pads were replaced with a continuous stationary cylinder with a half inch gap between the inner diameter of the cylinder and the outer diameter of the flywheel. U.S. Pat. No. 4,886,430 also assumed that vibration would be decreased or eliminated. Again, subsequent analysis showed that the rotor was dynamically unstable, most likely due to the relatively light unit loading and thick hydrodynamic film associated with such a large radial bearing which, if too thick of a film, causes the rotor to "wander" around within the bearing.
In view of the shortcomings of the flywheel radial-bearing arrangement of the above U.S. Pat. No. 4,886,430, it was decided by the personnel of Westinghouse Electric Corporation to provide a radial bearing having a smaller radius than that discussed in U.S. Pat. No. 4,886,430 with one-quarter to one-half inch radial clearance around the outer diameter of the flywheel. This entailed placing the radial bearing adjacent to the flywheel along the shaft.
A disadvantage of this arrangement was that the overall length of the motor was increased in view of the added length of the shaft and bearing housing accommodating the radial bearing. This increase in length of the motor results in an increase in plant costs due to the increase in the depth of the pit housing the pump and to the added inventory of the water, which must be provided inside the reactor containment in order to keep the reactor core covered in the event of a break in the pipes.
Ideally, a small diameter radial bearing and a greater clearance around the outer diameter of a flywheel while still maintaining the normal length of a canned motor pump would eliminate the problems associated with the prior art. This attempt is made in the parent case bearing Ser. No. 175,866 and filed on Dec. 30, 1993 by the present inventor and assigned to Westinghouse Electric Corporation for which the present application is a continuation-in-part application.
This arrangement for a radial bearing assembly of application Ser. No. 175,866 locates the radial bearing on the shaft inside the inner circumference of the flywheel rather than on the outside diameter of the flywheel as disclosed in the U.S. Pat. No. 4,886,430, or adjacent to the flywheel as discussed hereinabove.
In application Ser. No. 175,866, the flywheel has a stepped inner circumferential surface, and the shaft has an outer circumferential surface which may carry a radial journal. This arrangement allows a radial bearing assembly to be mounted inside the inner circumference of the flywheel. The radial bearing assembly is carried by a bearing housing member which also carries a thrust bearing assembly. The bearing housing member is stationarily mounted to an inner annular member which, in turn, is stationarily fixed to an outer housing for the motor of the canned motor pump. In one embodiment, the radial bearing assembly is mounted on the bearing housing member for bearing surface contact with the inner circumferential surface of the stepped portion of the rotary flywheel assembly. In another embodiment, the radial bearing assembly is mounted on the bearing housing member for bearing surface contact with a journal on the outer circumferential surface of the rotary shaft.
This latter embodiment of application Ser. No. 175,866, where the radial bearing assembly is inside the flywheel assembly for bearing surface contact with the outer circumferential surface of the rotary shaft in a canned motor pump involves the "conventional" kind of pivoted pad radial bearings in that the radial bearing has pivoted concave bearing pads that run on the outer diameter of a rotating shaft.
For bearing loads of a high inertia rotor where the rotating inertia is about 5000 lb.-ft.sup.2, a bearing diameter of about 91/2 inches, such as that of FIG. 3 of application Ser. No. 175,866 is adequate. However, in a canned motor pump where certain applications require a higher rotating inertia, of say about 10,000 lb.-ft.sup.2, a much longer and much heavier flywheel is needed. This would require a greater radial bearing diameter in order to support the subsequent increased bearing load.
Typically, an increase in the diameter of the radial bearing would require an increase in the inner diameter of the end of the flywheel, with a subsequent reduction in the rotating inertia.
There remains, therefore, when certain applications require an increase in the load capacity which, in turn, require an increase in the rotating inertia of a flywheel, a need not to increase the physical size of the radial bearings.